Electrically Erasable Programmable Read-Only Memories (EEPROM) are a kind of semiconductor memory device which has a minimum operation unit of byte, and can be electrically written repeatedly. Compared with Erasable Programmable Read-Only Memories (EPROM), information of the EEPROMs can be erased through a specific voltage without ultraviolet irradiation and dismantling, so that new data can be written. Because of excellent performances and conveniences for online operations, the EEPROMs are widely used in BIOS chips and flash memory chips which are erased frequently, and are gradually replacing parts of Random Access Memories (RAM), which have to retain data in power-off time, and even parts of hard disks. The EEPROMs and high-speed RAMs have been two most popular and fastest-growing storage technologies of the twenty-first century.
Flash memory is a kind of flash memory. A flash memory usually includes a decoding circuit, a control circuit and a storage array. The storage array of the flash memory includes a plurality of storage units arranged in rows and columns. FIG. 1 illustrates a cross-sectional diagram of two adjacent storage units in a conventional flash memory storage array. Referring to FIG. 1, each storage unit includes a substrate 10, a drain electrode 11, a source electrode 12, a floating gate FG and a word line WL. Specifically, the drain electrode 11 and the source electrode 12 are formed in the substrate 10, the drain electrode 11 is connected with a bit line BL on a surface of the substrate 10, the source electrode 12 is connected with a source line SL on the surface of the substrate 10, the gate electrode is disposed between the source line SL and the bit line BL and is connected with the word line WL, and the floating gate FG is disposed on a part of the surface of the substrate 10 between the word line WL connected with the gate electrode and the bit line BL connected with the drain electrode 11.
Conventionally, in a flash memory, there is only one row of memory units which share the same source line being used for data storage, which leads to low usage rate of the memory units. Too many idle memory units occupy a large space. Therefore, the flash memory may have a large size but a low programming speed.